The present invention relates to the cushioning of energy generated by bumpers in vehicles and in particular relates to an improved bumper assembly for vehicles as well as a vehicle comprising this bumper assembly and a method for protecting a vehicle from damage caused by impacts.
It is known in the motor car industry to provide bumpers at the front and at the rear of a vehicle in order to cushion the energy resulting from an impact and limit as far as possible any damage to parts of the vehicle and injury to passengers and pedestrians who may be involved in the impact.
At present, in particular, the attention of vehicle manufacturers is directed towards reducing the damage which a vehicle suffers following impacts at relatively low speeds, typically less than about 15–16 km/h. Insurance companies in certain countries, for example Germany and Great Britain, in an attempt to provide increasingly fairer policy premiums, have introduced new evaluation parameters. The current tendency is in fact that of offering a policy premium which is proportional to the so-called “insurance class” of the vehicle and not just the power and the value of the said vehicle. In turn, the insurance class is calculated on the basis of the cost of repair of a vehicle model following impact of both the front and the rear of the vehicle against a rigid carriage at 15–16 km/h.
Car manufacturers, in order to avoid the risk of a reduced market share due to the high operating costs of their vehicles, which are in turn penalized by high insurance premiums, are seeking efficient solutions which will enable them to minimize the damage to their vehicles following impacts at low speeds, typically less than about 15–16 km/h.
At present, car manufacturers provide a so-called “sacrificial element” (otherwise known as “crash box”) which, in the event of impact, cushions most of the impact energy, being deformed but preventing deformation of the vehicle chassis. In fact, any deformation of the chassis would result in decidedly high repair costs and unacceptably high insurance costs as a result.
Typically, a bumper assembly with a sacrificial element of the known type comprises a pair of crash boxes, a cross member, a cushioning element (for example made of foam or the like) and a bumper shield. In the bumper assembly of the abovementioned known type, the two crash boxes are fixed to the ends of two respective longitudinal members of the vehicle chassis by means of two respective plates. The cross member is joined to the opposite side of the crash boxes and extends continuously from one crash box to the other. The cushioning element made of foam or the like, when envisaged, is constrained to the outside of the cross member. The bumper assembly is moreover completed by a bumper shield having primarily aesthetic and aerodynamic functions. The cushioning element, for example made of expanded polypropylene or the like, is characterized by a marked capacity to return resiliently into the initial non-deformed position at the end of an impact acting on it at very low speeds (speeds less than 4–5 km/h) and occurring for example during parking manoeuvres.
Conventionally, both the crash boxes and the cross member are made of metallic materials, typically sheet steel or aluminium. Generally, the presence of a cross member allows the impact pressure to be spread in a more uniform and equally distributed manner. In terms of order of magnitude, the force acting on the crash boxes relative to the surface area of the crash boxes (with no cross member) is equal to about 20 N/mm2, but decreases to about 3–5 N/mm2 when the cross member is present, owing to the improved impact surface area. Such a pressure is compatible with the rigidity (1–6 N/mm2) of a deformable NCAP barrier (“impactor”) used for simulation of a real impact. Therefore, the need to provide a cross member between two crash boxes is determined firstly by the fact that, in the case of real impacts involving two cars, the cross member of the striking vehicle simulates the rigid impact of a so-called “insurance test”.
Moreover, the cross member ensures an improved distribution of the impact, even if offset. In fact, usually the barrier (or impactor) used to carry out the insurance impact tests covers 40% of the vehicle. For this reason, on account of the geometry of the bumpers, an offset impact does not occur against the boxes, but the first contact during impact occurs at about 2–3 cm from the latter.
Finally, in the case of an impact with a pole in a substantially central zone, the cross member allows the force to be distributed over the two lateral crash boxes for improved protection of the vehicle.
In the known solution described above, the cross member is fixed to the distal end of the crash boxes by means of a conventional mechanical fixing system, for example by means of welding, screws and bolts or the like. For the purpose of the present patent application, the adjectives “distal” (or “external”) and “proximal” (or “internal”) must be understood with reference to a vehicle (not shown in its entirety). In other words, the “distal” side is the side furthest from the vehicle, while the “proximal” side is the side closest to the said vehicle.
The known solutions described above (metal crash boxes, plates for fixing the crash boxes to the longitudinal members and metal cross members fixed to the distal ends of the crash boxes) are satisfactory from the point of view of the impact resistance, but are considered to be somewhat awkward to assemble, heavy, costly and not easily adaptable to new vehicle models.
Bumper assemblies comprising only plastic crash boxes (which perform essentially the same functions as metal crash boxes) are also known. These bumper assemblies do not have a continuous cross member, but nevertheless are able to pass the insurance impact tests carried out in laboratories. These bumper assemblies work reasonably well for impacts against fixed obstacles and may be made precisely of plastic materials, in a low-cost manner and with fairly low weights, for example by means of a plastic honeycomb structure.
However, a drawback of these bumper assemblies without cross member is the inability to withstand adequately concentrated central impacts, of the type known as “pole impact”.
Finally, in an attempt to solve the drawbacks of bumper assemblies without a cross member, compromise solutions of associating a sheet metal cross member with the known plastic crash boxes have been proposed. In the case of these bumper assemblies also, the sheet metal cross member is constrained to the distal end of the plastic crash box. These bumper assemblies offer a greater resistance to the concentrated central impacts than the solutions without cross member. The drawback in this case consists in the difficulty of constraining in an effective, lasting and reliable manner a fairly heavy metal body (cross member) to a plastic body (crash box, in turn fixed to the longitudinal member).
For example, according to a first known embodiment, the metal cross member was constrained to each of the plastic crash boxes by means of a through-screw system, but this had the negative result of a different deformational behaviour of the crash boxes. Basically, the through-screws, in the event of impact, tend to cut the crash boxes through.
According to a second known embodiment, the metal cross member was constrained to each of the plastic crash boxes by means of elongated projections which engage with corresponding through-holes in the cross member. The ends of the elongated projections which protrude from the holes of the cross members are then hot-riveted in order to achieve fixing between the crash box and cross member. However, this second known embodiment does not provide guarantees as to the reliability and duration over time of the crash box/cross member constraining system because of the weight of the cross member itself, the mechanical stresses and the fixing areas which are relatively small.
From FR 2 763 546 a bumper assembly for a vehicle is known. The bumper assembly according to FR 2 763 546 comprises a bumper shield 2, a cross member 3 and two local bumpers 4 to be fixed to a vehicle chassis 5. Each of the local bumpers 4 comprises an absorber element 6 and two “U” shaped plates 7. The plates 7 and element 6 form an elastic assembly arranged between the vehicle chassis and the cross member. The length of the “U” shaped plates is equal to the one of the absorber element. Thus, the cross member 3 is not fixed to the vehicle chassis in a rigid manner. Furthermore, such cross member is fixed to the chassis by metal plates, screws and bolts.
From U.S. Pat. No. 3,744,835 a shock absorbing honeycomb bumper is known. The bumper according to U.S. Pat. No. 3,744 835 is constructed from plastic, metallic or paper honeycomb material and covered by a rubber, metal or plastic shell. The bumper is shock absorbent to reduce damage to the fenders and body of the automotive during slow speed crashes. The bumper is attached to the vehicle by a pair of T-bolts which are fastened by nuts on fender. Thus, the bumper is not attached directly to the vehicle chassis and it does not provides for a rigid connection between a cross member and the vehicle chassis.
In the light of these not entirely satisfactory results of the known bumper assemblies, the Applicant has attempted to find a solution able to provide a bumper assembly which would be effective in all impact conditions, practical in terms of assembly, substantially modular so as to be easily adapted to new vehicle models, as well as having a long duration over time and being low-cost.
In a first attempt to achieve the abovementioned objects, the Applicant simply replaced the sheet metal cross member with a cross member made of plastic material, while maintaining a similar system for fixing together the plastic crash box and cross member so as to form essentially a beam (cross member) system resting on two points (plastic crash boxes). According to a first attempt, the plates were simply used to constrain the crash boxes to the longitudinal members.
The Applicant however ascertained that, owing to the well-known low Young's modulus and yield point values of plastic materials (about 2,000 N/mm2 for non-reinforced plastics or in any case ≦10,000 N/mm2 for plastics reinforced with glass fibres compared to 210,000 N/mm2 for steel), with relatively small forces there is an excessive deformation in the cross member and therefore little energy cushioning. This behaviour does not guarantee protection of a vehicle in the event of an impact.
In order to improve these poor values for the Young's modulus and yield point, the Applicant used for the cross member a plastic material reinforced with short or long glass fibres. The Applicant established, however, that this increase in the Young's modulus and yield point values is accompanied in an undesirable manner by an increased fragility. For this reason, an impact force close to the maximum admissible value results in breakage of the cross member and therefore the situation where the two crash boxes are no longer connected to each other (reproducing a situation similar to the solution without cross member).
The Applicant obtained the same unsatisfactory results also when the cross member (made of non-reinforced or reinforced plastic material) is constrained to the bumper shield.
The Applicant realised that optimum results could be obtained using a different type of system for constraining cross member and crash boxes (absorbers), replacing a configuration of the type where the beam rests on two points with a configuration of the type where the beam is joined in an interfitting manner at two points.
In other words, the idea forming the basis of the present invention is that of constraining a plastic cross member, by means of a substantially rigid interfitting joint, to the chassis (typically to the two longitudinal members) of the vehicle, inserting the crash boxes inside it.
According to a first aspect, the present invention relates to a bumper assembly for a vehicle comprising a chassis. The bumper assembly comprises: at least two crash boxes; a cross member made of a first plastic material which, during use, extends over a distance at least corresponding to that between the crash boxes; and at least two substantially indeformable connecting devices connected to the cross member for rigidly fixing said cross member to the chassis of the vehicle, wherein the substantially indeformable connecting devices are connected to the cross member at positions corresponding to the crash boxes.
Typically, the substantially indeformable connecting devices are made of a second plastic material.
Profitably, the first plastic material is the same plastic material as the second plastic material.
Preferably, the substantially indeformable connecting devices are directly connected to the cross member by a welding process, more preferably by an hot blade welding process.
Preferably, both the first and second plastic materials comprise XENOY®.
According to a preferred embodiment, the at least two substantially indeformable connecting devices are substantially solid. Alternatively, the at least two substantially indeformable connecting devices have a honeycomb cross-section.
Advantageously, the cross member comprises at least one first connecting surface and each of said substantially indeformable connecting devices comprises at least one corresponding second surface for surface connecting said cross member to said substantially indeformable connecting devices along said surfaces.
Preferably, the cross member has a cross-section which is substantially at least partially open in the form of an “Ω” or “H”.
Advantageously, each of said substantially indeformable connecting devices comprises a fixing device for fixing said substantially indeformable connecting device to a respective crash box.
According to a preferred embodiment, each of the crash boxes is integral with a respective substantially indeformable connecting device. Alternatively, the crash boxes are integral with said cross member.
The bumper according to the present invention could further comprise one or more of: a reinforcing element, an impact cushioning element, a bumper shield.
According to a second aspect, the present invention provides a method for protecting a vehicle from damage caused by impacts, said vehicle comprising a chassis, the method comprising the steps of: providing at least two crash boxes; providing a cross member made of a first plastic material which extends over a distance at least corresponding to that between the crash boxes; and directly connecting to said cross member at least two substantially indeformable connecting devices for rigidly fixing said cross member to said chassis of the vehicle, wherein said at least two substantially indeformable connecting devices are connected to the cross member at positions corresponding to the crash boxes.
According to a third aspect, the present invention provides a vehicle with a chassis and comprising a bumper assembly which comprises: at least two crash boxes; a cross member made of a first plastic material which, during use, extends over a distance at least corresponding to that between the crash boxes; and at least two substantially indeformable connecting devices connected to the cross member for rigidly fixing said cross member to the chassis of the vehicle, wherein the substantially indeformable connecting devices are connected to the cross member at positions corresponding to the crash boxes.